Crystalline structures come in various sizes and forms. Many useful devices are formed as thin crystalline films, typically formed upon a substrate. Examples of such films include thin film high temperature superconductors formed upon substrates, thin magnetic films, such as ferroelectric films formed upon substrates, or optical materials, such as when non-linear optical materials are interfaced with semiconductor devices.
Ordinarily, the decision of what substrate material to use with a crystalline film has been subject to severe restrictions. Generally, the substrate was chosen so as to optimize the growth prospects of the film. Therefore, factor such as minimizing cost or providing good performance properties for the intended use of the ultimate film were not emphasized.
While there are many considerations in choosing the substrate, the following factors are important. They are not necessarily listed in order of importance.
First, the thermal expansion properties of the substrate and the crystal must be compatible. High temperature processing of crystalline films typically require that the substrate have thermal expansion properties similar to that of the crystal, least the crystal "crack" upon cooling of the substrate and crystal.
Second, the substrate must have good thermal stability properties. Crystal processing often occurs at many hundreds .degree.C., often up to 900.degree. C., and therefore, require that the substrate be stable throughout the processing temperature range.
Third, there must be a relatively good match of the crystal lattice structure between the substrate and the film. If the lattice mismatch is too great, the crystal and substrate will not been an epitaxial relationship which is required for the best properties.
Fourth, interdiffusion between the substrate and the film must be considered. If the substrate is such that there is substantial interdiffusion between the substrate and the film, suboptimal films may result.
Fifth, the fragility of the substrates must be considered. One function of the substrate is to provide support to the thin film. If the substrate itself is fragile, the overall structure may have unacceptably high fragility.
Sixth, the various structures of the substrate must be considered. One particularly problematic crystal phenomenon is that of twinning, wherein the crystal changes structure as a function of temperature. Films which are formed at a temperature above the twinning temperature often become unacceptable at a lower temperature, when the substrate is in a non-twinned state.
Seventh, the dielectric constant of the substrate must be considered. Often, substrates which are acceptable for other reasons have an unacceptably high dielectric constant. This is often a concern for microwave compatible devices as the dielectric constant of the substrate strongly impacts the overall dielectric constant of the device.
Finally, the cost of the substrate is often significant. Except with very common materials such as silicon, single crystal substrate materials tend to be very expensive. Alternatively, polycrystalline materials tend to be relatively inexpensive.
Thus, when making a choice of a substrate, the historical emphasis was on compatibility of the substrate with the growth of a quality film. For example, the concerns of thermal expansion, thermal stability, lattice mismatch, interdiffusion and twinning were of concern. The factors such as dielectric constant and cost were of lesser importance. Significantly, however, these factors are critically important to formation of useful devices, and ultimately, market acceptance.
Various high temperature superconducting thin film structures have been known now for some time. Since there discovery in 1986 by Bednorz and Muller, high temperature superconducting materials have been formulated in different forms. While early materials were formed in generally bulk form, useful devices have been fabricated from thin film materials. Olson, et al, "Preparation of Superconducting TICaBaCu Thin Films By Chemical Deposition", Appl. Phys. Lett. 55, No. 2, 189-190 (1989).
Since the discovery of high temperature superconductivity in 1986 with the discovery of BaLaCuO systems, (see "Possible High T.sub.c Superconducting in the Ba-La-Cu-O System", Bednorz and Mueller, V. Phys. B-Condensed Matter No. 64, 189-193 (1986)), which was followed by the discovery of the YBCO materials, (see Wu et al, "Superconductivity at 93K in a New Mixed-Phase YBaCuO Compound System At Ambient Pressure", Physical Review Letters, Vol. 58, No. 9, pp 908-910 (1986)), followed by the discovery of the bismuth materials, (see Maeda et al, Japanese Journal of Applied Physics, Vol. 27, No. 2, pp L209-210 (1988) and Chu et al, "Superconductivity up to 114K in the BiAlCaSrCuO Compound System Without Rare-Earth Elements", Physical Review Letters, Vol. 60, No. 10, pp 941-943 (1988)) and the thallium based system such as TlCaBaCuO and TlBaCuO, see Sheng and Hermann Nature, 332: 55-58 (1988) and Sheng and Hermann, Nature 332: 138- 139 (1988). Generally, these materials all include layered copper oxide planes. They will be referred to as a class as the "layered copper oxide" materials. Often times, these materials contain substitute substituent elements. The typical substrates of choice for the various layered copper oxide materials include: LaAlO.sub.3, MgO, SrTiO.sub.3, yttria stabilized zirconia and sapphire.
Considering high temperature superconductors further, various applications have been described for such devices, such as use in microwave systems, in multichip modules, for magnetic resonance imaging coils, as superconductive quantum interference devices (Squids), as infrared detectors and as junctions for logic. However, these devices have been limited in their theoretical performance by the non-superconducting properties of the other material, such as the substrates. For example, in the case of microwave devices, a substrate such as SrTiO.sub.3 on which a superconducting line of YBCO or thallium has been formed may have substantial advantages over non-superconducting devices, the SrTiO.sub.3 substrate still has a dielectric constant of 150 or greater. This adds to the overall electrical environment of the superconductor, causing suboptimal results. However, SrTiO.sub.3 has proved to be a substrate of choice for layered copper oxide films as they have good lattice match to film such as YBCO, differing only by 1.2%, have good thermal stability, and have thermal expansion properties similar to various layered copper oxide materials. Thus, the choice of a substrate is once again dictated by growth requirements, even though the substrate may have suboptimal properties for intended applications.
In the field of semiconductors, it is a known desirable goal to form ferroelectrics and non-linear optical devices on semiconductors. In the case of ferroelectrics, non-volatile memories may be formed by forming the ferroelectric directly upon the semiconductor. For optical devices, it is desirable to connect devices such as fiberoptics to semiconductor devices. To date, efforts to form ferroelectrics such as BaTiO.sub.3 on silicon have proved unsatisfactory, principally because of interdiffusion between the film and the substrate, because of differential thermal expansion, causing unacceptable film cracking.
Various attempts have been made to overcome the problem of substrates which are not optimally matched to use and commercialization of the films. In the semiconductor area, gallium arsenide ("GaAs") films have been grown with aluminum arsenide ("AlAs") as a sacraficial layer. Aluminum arsenide has a preferential etch rate of 10.sup.8 times larger than that of gallium arsenide. Hydrofluoric acid is used to etch away the sacraficial layer. However, the use of hydrofluoric acid is not desirable, given its hazardous properties. Further, gallium arsenide and aluminum arsenide are not compatible necessarily with oxide films, such as the layered copper oxides or the materials upon which they are to be formed.
An alternative solution which has been used both in the field of semiconductors and superconductors is that of buffer layers. Generally, a buffer layer is an intermediate layer disposed between the substrate and the desired film. Buffer layers are advantageous in that they tend to prevent interdiffusion between the substrate and the film and tend to grade the lattice mismatch, assuming the buffer layer is properly chosen. The disadvantages of use of a buffer layer include: thermal expansion problems, cost, dielectric problems associated with the substrate. Further, buffer layers tend to be very specific solutions to the combinations of substrate and film, thus not providing an acceptable general solution.
Despite the long-standing and vexing nature of this problem, no satisfactory solution has been proposed heretofore.